Coating compositions for magnetic materials



Patented Jan. 3, 1950 COATING COMPOSITIONS FOR MAGNETIC MATERIALS Gerald W. Young, Schenectady, N. Y.. assignor to General Electric Company, a corporation of New York No Drawing. Application April 19, 1947, Serial No. 742,727

7 Claims.

This invention relates to coating compositions for magnetic materials, and more particularly, to coating compositions for magnetic materials comprising a drying-oil modified phenolic resin and silica aerogel.

In the construction of electrical machinery having laminated magnetic cores such as, for example, generators, motors, and transformers, it is essential that the individual laminations of magnetic material be insulated from the adjacent laminations in order to avoid short circuiting within the core structure. Insulating compositions for this purpose should be resistant to flow or appreciable deformation at elevated operating temperatures, electrically stable even when subjected to conditions of high humidity, and should desirably be capable of withstanding temperatures up to from 200' C. to 250 C. for ordinary overload periods without decomposition, or burning through, thus. causing a short circuit from one magnetic lamination to an adjacent lamination. This last-mentioned requirement is particularly important in large generators and motors where the core mass is large and difficult to cool, and local overheating may occur during overload periods. In addition to the aforementioned properties, it is important, from the standpoint of manufacture, that the insulating material be in such form prior to application that it may readily be applied to the surface of the magnetic material and, further, that laminations coated with the composition be capable of withstanding handling during fabrication and assembly operations.

Prior to this invention various compositions have been employed to electrically insulate magnetic core laminations. In order to avoid, or at least reduce, failures due to operation at elevated temperatures such as might result during extended overload periods, inorganic insulations such as metallic oxides and sodium silicate have been applied to magnetic core laminations prior to assembly of the cores. However, difficulty has been experienced in the use of these inorganic materials because, in general. thin coatings of such inorganic materials do not adhere readily to the magnetic material, and tend to be easily damaged during the handling of the laminations incident to assembly of the core. In order to overcome the fragility of these coatings they have in some cases been applied to paper, the combination of paper and inorganic material then being used as the insulation between the magnetic laminations. It is obvious, however, that the use of paper introduces a possible source of failure because of the tendency of paper to decompose and carbonize in spots where overheating occurs. Furthermore, if free alkali is present in the inorganic material, the insulating coating picks up moisture during periods of high atmospheric humidity, causing appreciable reduction of its insulation resistance.

Other insulating compositions which are superior to the above-mentioned inorganic coatings from the standpoint of ease of applicationof uniform coatings, adherence to the magnetic laminations, and resistance to damage during assembly operations have been prepared from various synthetic and natural resins. Among these compositions are those containing synthetic resins of the phenolic type. Attempts have been made to improve the resistance of these resinous compositions to elevated temperatures by the addition of various inorganic fillers. difiiculty has been experienced in obtaining compositions of this type which can be applied to the close tolerances required for insulation between laminations of magnetic cores. In addition, the filler tends to settle out of the liquid composition in the reservoirs of the coating apparatus.

The compositions of my invention which comprise a drying-oil modified phenolic resin and a finely divided form of silica commonly known as silica aerogel, afford all the advantages of the above-mentioned resinous insulation coatings and, in addition, are electrically stable under conditions of high humidity and are capable of withstanding elevated temperatures up to from about 200 C. to about 250 C. during overload periods without breaking down and causing short circuiting of magnetic laminations at the hot spot.

Furthermore, these compositions can readily be applied to close tolerances and there is no tendency for the silica aerogel to settle out of the liquid composition during the application of the coating to the magnetic laminations.

The drying-oil modified phenolic resins which are a component of my composition are prepared by adding an oil-soluble resinous reaction product of an ortho or para-substituted phenol having two reactive positions in the aromatic nucleus, and an aliphatic aldehyde to a drying oil, (e. g., linseed, tung, perilla, cashew nut, oiticica and soya oils), which has been heated to a sufficiently high temperature (e. g., 175-225 C.) to facilitate the melting and dissolving of the resin. The resulting reaction mixture is then reheated to the desired temperature as hereinafter described and allowed to cool slowly until the desired cure rate is obtained. The length of time and temperature employed in heating the mixture of oil and resin and However, in general,

- the resulting cure rate of the oil-modified resin may be widely varied depending upon the particular requirements for the final insulating composition of which the oil-modified resin is a component. For example, the mixture of oil and resin may be maintained at temperatures from about 100 C. to about 200 C. until a small pill of the resulting reaction mass cures on a hot plate at 200 C. in from about 30 to about 50 seconds. I have found that particularly good results are obtained when the mixture is cooked until the cure rate is approximately 40 seconds at 200 C. It will be obvious that a longer period of cooking is required, but that more accurate control is possible, when lower cooking temperatures are employed.

Illustrative examples of ortho and para-substituted phenols which may be employed in the preparation of the above oil-soluble resins are the cresols, xylenols, p-tertiary butyl phenol, p-tertiary amyl phenol, p-phenyl phenol, and other ortho and para-substituted hydrocarbon radicalsubstituted phenols, such as, for example, cyclohexyl phenol. The choice of the aliphatic aldehyde component is dependent largely upon economic considerations and upon the particular properties desired in the finished product. I prefer to use formaldehyde as the aldehydic component. However, other aliphatic aldehydes, for example, acrolein, acetaldehyde, propionaldehyde, butyraldehyde, etc., may be employed,

Oil-soluble resins prepared from the phenolic bodies and aldehydes set forth above are well known in the art and may be prepared, for example, as disclosed in DAlelio Patent No. 2,337,873, assigned to the same assignee as the present invention.

Various proportions of resin to oil may be employed in preparing the above-described oil-modifled resin component of the coating compositions. However, it is preferable to use a mixture having a ratio by weight of 0.7 to 1.5 parts of oil to one part of resin.

After the mixture of drying oil and oil-soluble resin has been reacted until the resulting oilmodified resin has the desired cure rate, the mass is allowed to cool sufficiently to permit the addition of thinners. In general, hydrocarbons having boiling ranges between about 80 C. and 200 C. may be employed as thinners. The solvent or mixture of solvents employed should not be so volatile as to be readily volatilized during the addition of the thinner to the oil-resin reaction mass. Examples of such hydrocarbons are: mono-cyc1ic aromatic hydrocarbons including benzene, toluene, xylene, etc., and petroleum hydrocarbons such as gasoline, kerosene, V. M. and P. naptha, heptane, etc. The amount of thinners added may be widely varied, but in general, I prefer to add thinners in amounts from about 0.75 part to 1 part by weight of thinner t 1 part of oil-modified resin. After the oil-modifled resins have been completely dissolved in the thinner or mixture of thinners a small amount of driers, for example, about 2 per cent by weight of the oil-modified resin are added to the resin solution. Suitable driers for this purpose are, for example, lead, cobalt and manganese napthanates.

In accordance with my invention, a finely divided, porous, -friable form of silica commonly known as silica aerogel is added to the solution of drying-oil modified resin prepared as described above. Silica aerogel is prepared by a method well known in the art, involving the evaporation of liquid from a silica gel under controlled conditions of pressure and temperature. The

amount of silica aerogel which may be incorporated in my compositions may be varied from about 20 per cent to about 60 per cent of the weight of the oil-modified phenolic resin present in my compositions. Thus the final insulating coatings may contain from about 17 per cent to about 37 per cent of silica aerogel depending upon the requirements of the particular application. In order to thoroughly incorporate the aerogel with the resin solution, the mixture is preferably ground in a ball mill for about 12 to 24 hours.

In order that those skilled in the art better may understand how the present invention may be carried into efl'ect, the following example thereof, which I have found to produce particularly good results, is given by way of illustration. All parts are by weight.

Resin base: Parts Para tertiary butyl phenol-formaldehyde resin 24.2 Aged linseed oil 29.1 Thinners:

Aromatic naphtha 20.73 Kerosene 20.79 Driers:

Lead drier (24% solution in mineral spirits) 4.85 Manganese drier (6% solution in mineral spirits) .33 Silica gel 17.1

The linseed oil is loaded into a kettle and heated to between 200 C. and 205 C. Heating is then stopped and the para tertiary butyl phenolformaldehyde resin, which has been previously broken into small pieces is added to the oil. The mixture in the kettle is stirred vigorously, and when the resin is completely melted, heating is again started. The temperature of the mass is raised to about 170 C. and heating is again stopped. The reaction between the resin and the oil is allowed to continue until the cure rate of the reaction mass at 200 C. drops to about 40 seconds. The thinners are added with stirring after allowing the oil-modified resin mass to cool sufilciently to avoid excessive evaporation of the thinners during the addition. The silica aerogel is then thoroughly mixed and ground with the resin solution in a ball mill for about 24 hours.

Various procedures may be employed in the application of the compositions of my invention depending on the design and performance requirements of the particular magnetic core lamination to be insulated. In general, the following procedure affords the desired results.

After the core lamination has been fabricated to the desired shape, a coating of the insulating composition may be applied by any of the wellknown methods such as, for example, by roller coating or by spraying, brushing, or dipping, to the desired thickness (e. g., 0.1 to 1.0 mils). The coated lamination is then subjected to'lieat in order to evaporate the solvents from the coating and to cure the residue of oil-modified resin containing the silica aerogel. Various temperatures and times of cure may be employed in this step depending on production requirements. However, I have achieved particularly good results by passing the coated laminations through a continuous oven which provides a temperature range varying from about C. at the entrance to about 300 C. at the exit of the oven. The desired cure is accomplished by passing the coated lamination through the oven in about 30 seconds. After the coating is cured, the lamination is ready for assembly into the core structure.

Many failures of electrical equipment in the field, particularly where large generators are concerned, have been traceable to hot spots and short circuits within the core structure because of overheating and failure of the insulation between the core laminations. The coating compositions of my invention are characterized by an ability to afford effective insulating properties even after the core has been heated to such extreme temperatures that the drying-oil modified resin binder is destroyed. This property is attributable to the unique characteristic of the silica aerogel which is its ability to adhere to the magnetic material even after the resinous binder has carbonized and burned ofl. To illustrate this property of the aerogel. a number of strips of steel core laminations were coated with resinous insulating compositions containing 25 per cent of various inorganic fillers in the coating after the baking operation. After burning away the resin on all of the laminations by heating at 350 C.

for 64 hours, the silica aerogel was the only filler which remained adherent to the steel. The other fillers could be readily dusted from the surface of the steel.

To further illustrate the marked improvement in resistance breakdown at elevated temperatures which is characteristic of my aerogel-filled coating compositions, the following comparative test was made:

Thirty-five magnetic steel strips (1%" x x 14 mils) were coated on both sides with a dryingoil modified phenolic varnish identical to the composition prepared as described in the above example except that it contained no silica aerogel. Thirty-five additional strips were coated with the composition prepared as described in the example, including the silica aerogel filler. All the strips were baked and the two sets of thirty-five strips each were assembled into two stacks of about /2 inch thickness between heavy steel plates held together by bolts. A compressive stress of 200 pounds per square inch was applied to the stacks in a press and the bolts were turned up tight to hold the laminations under compression. The bolted stacks were placed in an oven at 250 C. and the resistance when a potential of 3 volts was applied between the outer laminations of each stack was periodically determined. The resistance of the stack which contained no silica aerogel in the insulation between the lamination had dropped below 100 ohms at the end of 200 hours. The stack of strips insulated with the composition of my invention still had substantially its original resistance at the end of 200 hours and at the end of 10,000 hours, showed a resistance of 10,000 ohms.

A further advantage in the use of silica aerogel as a filler in my compositions lies in its ability to remain suspended in the composition for a long period of time. For example, after standing for two weeks the silica aerogel in my compositions was still in colloidal suspension in the resin solution. In similar compositions in which various other fillers such as silica gel, red iron oxide, and alumina were employed, the fillers had settled out at the end of the two weeks. This suspension phenomena can be partly explained by the extremely fine particle size and the low specific gravity of the particles of aerogel.

Because of the extreme fineness oi. the aerogel particles and the good heat transfer properties of my coating compositions, it is possible to apply the desiredthickness of insulation on core laminations to close tolerances and obtain a uniform degree of cure in a single coating operation. Heretofore, in employing the oil-modified resin composition without the aerogel filler, it has been necessary to apply and bake the coating in two layers.

What Iclaim as new and desire to secure by Letters Patent of the United States is:

1. An insulating composition for coating laminations of magnetic cores consisting essentially of i 1) a mixture of a drying oil-modified phenolic resin consisting by weight of from 0.7 to 1.5 parts drying oil to one part of the oil-soluble resinous reaction'product of an aldehyde and a phenol of the class consisting of ortho hydrocarbon-substituted and para hydrocarbon-substituted phenols, (2) a hydrocarbon solvent for said resin boiling between about 80 C. and 200 C., said solvent being present in an amount from about 0.75 to 1 part per part of oil-modified resin, and (3) from to per cent silica aerogel based on the weight of the oil-modified resin.

2. The composition of claim 1 in which the aldehyde is formaldehyde.

3. The composition of claim 1 in which the phenol is para-tertiary butyl phenol and the aidehyde is formaldehyde.

4. The composition of claim 1 in which the drying oil-modified resin is a linseed oil-modified para-tertiary butyl phenol-formaldehyde resin.

5. An insulating composition for coating laminations of magnetic cores consisting essentially of (1) 53.3 parts by weight of linseed oil-modifled para-tertiary butyl phenol-formaldehyde resin containing about 29 parts oil and 24 parts resin, (2) about 41.5 parts of a hydrocarbon solvent for the resin of (1) and boiling between and 200* C. and (3) about 1'7 parts by weight of silica aerogel.

6. Electrical equipment including a laminated magnetic core the laminations of which are coated with heat hardened insulating composition consisting essentially of 1) a mixture of a drying oil-modified phenolic resin consisting by weight of from 0.7 to 1.5 parts drying oil to one part of the oil-soluble resinous reaction product of an aldehyde and a phenol of the class consisting of ortho hydrocarbon-substituted and parahydrocarbon-substituted phenols, and (2) from 20 to 60 per cent silica aerogel based on the weight of the oil-modified resin.

7. Electrical equipment including a laminated magnetic core, the individual laminations of magnetic material being coated with an insulating coating consisting essentially of (1) 53.3 parts by weight of linseed oil-modified para-tertiary butyl phenol-formaldehyde resin containing about 29 parts oil and 24 parts resin, and (2) about 1'7 parts by weight of silica aerogel.

GERALD W. YOUNG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

6. ELECTRICAL EQUIPMENT INCLUDING A LAMINATED MAGNETIC CORE THE LAMINATIONS OF WHICH ARE COATED WITH HEAT HARDENED INSULATING COMPOSITION CONSISTING ESSENTIALLY OF (1) A MIXTURE OF A DRYING OIL-MODIFIED PHENOLIC RESIN CONSISTING BY WEIGHT OF FROM 0.7 TO 1.5 PARTS DRYING OIL TO ONE PART OF THE OIL-SOLUBLE RESINOUS REACTION PRODUCT OF AN ALDEHYDE AND A PHENOL OF THE CLASS CONSISTING OF ORTHO HYDROCARBON-SUBSTITUTED AND PARAHYDROCARBON-SUBSTITUTED PHENOLS, AND (2) FROM 20 TO 60 PER CENT SILICA AEROGEL BASED ON THE WEIGHT OF THE OIL-MODIFIED RESIN. 